Honestly, I never saw guests "fitting" as something necessary. Do I prefer someone like Geralt of Rivia or Ezio Auditore in Soul Calibur over Darth Vader and 2B? Absolutely, but as much as I care about a fighting game's story and setting (which is more than can be said about a lot of people, who dismiss it as "irrelevant"), no "unfitting" character ever broke my immersion by existing within the game, specifically because those characters rarely are canon, hell, the only guest character who I think is legitimately canon in a fighting game is Akuma in Tekken 7 (and he is kinda like an alternative reality version of the original), even those who have story modes, like Geralt in Soul Calibur 6, aren't canon, they feel more like something in the outside, a what-if, an add-on. I actually WOULD be upset if they tried to mix up some of those characters in the actual canon, like adding an actual Terminator in the canon story of Mortal Kombat would absolutely piss me off. I don't know the details about Akuma being added to Tekken, except for the fact that its like an alternative reality thing, which, depending on how its done, could work well for Akuma, or hell, even a cross between Soul Calibur and Tekken (which is technically already a thing, since the Yoshimitsus from Soul Calibur are, canonically, predecessors of the ones from Tekken, which makes me wonder how all those crazy powers from the former game just don't show up in the latter's timeline). Even Street Fighter managed to attach itself to Final Fight, Rival Schools, most recently Strider and I believe even Captain Commando and Street Fighter 2010 are included, so its not like we haven't seen weird canon stuff, but even then, there are some limits.